This application is related to Japanese Patent Application No. 2001-383178 filed in Dec. 17, 2001, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a magneto-optical recording medium, and more particularly, it relates to a magneto-optical recording medium, in which recordation and reproduction of information are carried out by incident of light from the opposite side to the substrate.
2. Description of the Related Art
Most conventional magneto-optical recording media are formed with a substrate of a thickness of about 1.0 mm having accumulated thereon a recording layer and a protective layer, and recording and reproduction of information are carried out by the incidence of light through the substrate.
As means for developing a high density magneto-optical recording medium, a spot size of a light beam irradiated on a magneto-optical recording medium has been decreased. In general, there is a relationship expressed by the equation xcfx86=xcex/2NA, in which xcfx86 represents the spot size, NA represents a numerical aperture of an objective lens, and xcex represents a wavelength of the laser light. According to the equation, in order to decrease the spot size xcfx86, it is necessary to increase the numerical aperture NA of the objective lens. When NA is increased, the focal length is shortened while the resolution can be increased.
Therefore, when the numerical aperture NA becomes larger, the aberration is increased due to thickness unevenness and tilt of the substrate, and thus it is necessary to decrease the thickness of the substrate as far as possible. Accordingly, it is preferred that recordation and reproduction are carried out by incident of light from the side of the recording layer for realizing high density recording, rather than recordation and reproduction that are carried out by incident of light from the side of the substrate.
The model of recordation and reproduction carried out by incident of light from the side of the recording layer is hereinafter referred to as a front illumination model.
FIG. 9 is a schematic cross sectional view showing a conventional magneto-optical recording medium of the front illumination model.
The magneto-optical recording medium contains a substrate 1 formed, for example, with polycarbonate having accumulated thereon at least a reflective layer 2, a recording layer 4, a protective layer 5 and a coating layer 6. The reflective layer 2 is generally constituted, for example, with a metallic film, such as silver, and reflects a light beam 7 incident from above on the coating layer 6 to the side of the coating layer 6.
The reflective layer 2 is demanded to have, in addition to the function of reflecting light, a function of heat liberation for recording marks of refined shapes on the recording layer. Therefore, it is necessary in general that the reflective layer 2 has a thickness of about 100 nm or more.
The reflective layer 2 is formed on the substrate 1 by a DC sputtering method using a solid metal target, such as silver.
However, in the case where a reflective layer having a thickness of 100 nm or more is formed, uneven convexoconcaves of granular form of about 50 nm or more are formed on the surface of the reflective layer 2. For example, lands and grooves having a width of about 0.2 xcexcm are formed in a convexoconcave form on the surface of the substrate 1. When the reflective layer 2 having uneven convexoconcaves of granular form is formed on the lands and the grooves, such a problem occurs mainly in that the lands are broadened to dull up the corners of the rectangular shape, whereby the predetermined land-groove width ratio cannot be observed.
Ra is used as a parameter for showing surface roughness of a substrate. While the surface roughness Ra of the substrate 1 itself is as small as about 0.3 nm, the surface roughness Ra is increased to more than 1.5 nm when the reflective layer 2 of about 100 nm is accumulated. When the surface roughness is increased by forming the reflective layer 2, media noise is increased, which adversely affects the magnetic characteristics of the recording layer 4 formed on the reflective layer 2, whereby high resolution cannot be realized.
In order to improve the surface roughness of the reflective layer 2, it is considered that the reflective layer 2 after forming is subjected to an etching treatment to be smoothened. Furthermore, it is also considered that an alloy having an additive doped thereto (such as Si doped Ag) is used as a material for forming the reflective layer 2 to improve the smoothness with the function of heat liberation being maintained.
When the smoothness of the reflective layer 2 is improved, the so-called media noise is decreased to improve CNR and SNR, but when the surface of the reflective layer 2 does not have an appropriate roughness, such a problem arises that the coercive force Hc of the recording layer 4 formed thereon is lowered.
The lowering of the coercive force Hc brings about deterioration of recordation and reproduction of information recorded by a temperature or a magnetic field, which are externally applied. Therefore, the smoothening of the reflective layer 2 is preferred for decreasing the media noise, but excessive smoothening is not preferred since the coercive force of the recording layer 4 is lowered.
The coercive force of the recording layer 4 is increased when it grows in such a manner that the directions of magnetization are aligned to one direction. For example, the directions of magnetization are liable to be aligned and are effective to increase the coercive force in the case where the surface of the reflective layer 2 as the underlayer has periodical convexoconcaves or grains of several tens nm, rather than in the case where substantially no convexoconcave is formed thereon.
However, when the period of the convexoconcaves formed on the surface of the reflective layer 2 is too large, short marks cannot be recorded, and as a result, high density recording cannot be carried out.
It is understood from the foregoing that the reflective layer 2 formed on the substrate necessarily has an appropriate thickness for maintaining the heat liberation function; necessarily has an appropriate surface roughness for aligning the directions of magnetization of the recording layer 4 to a certain direction to obtain a large coercive force; and preferably has a small period of the convexoconcaves formed on the surface for recording marks as short as possible to realize high density recording.
This invention provides a magneto-optical recording medium, in which plural reflective layers are provided, and the surface roughness and the surface tension of the respective reflective layers are appropriately selected, whereby the heat liberation characteristics of the reflective layers can be maintained, and the coercive force and the CNR can be improved.
The invention relates to, as one aspect, a magneto-optical recording medium comprising at least a reflective layer and a recording layer formed in this order on a substrate, wherein the medium is adapted to record and reproduce information by irradiation with light from a recording layer side, and the reflective layer comprises two or more thin film layers different in surface roughness, and of the thin film layers, one near the substrate has a surface tension smaller than a surface tension of one near the recording layer.
According to an aspect of the invention, the coercive force of the recording layer can be increased in comparison to the conventional products with the heat liberation characteristics of the reflective layer being maintained.
Furthermore, the CNR (carrier/noise ratio) can be improved, in addition to the reduction of the media noise, and thus the recording density can be increased.
It is possible that the thin film layers constituting the reflective layer are gradually adjusted in surface roughness in such a manner that the thin film layer nearest to the substrate has the smallest surface roughness, and the thin film layer nearest to the recording layer has the largest surface roughness. According to the embodiment, the coercive force of the recording layer can be further increased.
It is also possible that of the thin film layers constituting the reflective layer, the first thin film layer nearest to the substrate comprises Ag, Al or Ni as a main component and at least one element selected from Pd, Cu, Si, Ti, P and Cr added in prescribed amounts.
It is also possible that the other thin film layers than the first thin film layer comprise a material containing at least one element selected from W, Mo, Ta, Fe, Co, Ni, Cr, Pt, Ti, P, Au, Cu, Al, Ag, Si, Gd, Tb, Nd and Pd.